This invention relates to offset coils and more particularly to aluminum offset coils having a predominantly recrystallized globular and subsequently cold-deformed structure.
In the production of offset printing plates, rolled coils and sheets of aluminum or aluminum alloys having a thickness in the range of about 0.1 to 0.5 mm are used. In subsequent processing, the aluminum coil is provided with a photo-sensitive layer which is transformed by the action of light, thus rendering it insoluble in the developers used. By the end of the developing stage, an uncoated aluminum surface having hydrophilic properties is created in the unexposed areas of the coil.
The developed printing plate is clamped on a plate cylinder for the printing process. It is desired that the offset printing plates possess sufficient strength and resistance to crack formation, when subjected to mechanical stress.
To obtain a print edition of the highest possible quality, the rolled surface must be roughened either mechanically, electrochemically or chemically for better adhesion between the coil and the photo-sensitive layer. The roughening should be conducted so as to achieve a homogeneous and fine-celled surface.
To increase the quality of the color print edition, it if further desired to subject the developed support layer to a thermal process which results in the hardening of the remaining layer parts (image carrier). This thermal treatment is carried out at such temperatures that a distinct decrease in the strength of the aluminum is often brought about.
The above-described requirements for an offset coil could heretofore be met only with expensive technology. According to DE-OS No. 19 29 146 it has been proposed to clad a layer of pure aluminum on both sides of a core made of a pliant aluminum-magnesium-manganese alloy of high tensile strength. The cladded aluminum layer has a thickness of 2-16% (based on the total thickness of the core material) on each side of the core. This process provides enhanced strength properties along with favorable behavior on roughening. However, this process is extremely expensive.